This picture, like other members of the series, draws upon the version of the Perseus legend that appears in William Morris's "The Doom of King Acrisius" from The Earthly Paradise. See the relevant passage below:

May I not see this marvel of the lands
So mirrored, and yet live? Make no delay,
The sea is pouring fast into the bay,
And we must soon be gone."
"Look down", he said,
"And take good heed thou turnest not thine head."
Then gazing down with shuddering dread and awe,
Over her imaged shoulder, soon she saw
The head rise up, so beautiful and dread,
That, white and ghastly, yet seemed scarcely dead
Beside the image of her own fair face,
As, daring not to move from off the place,
But trembling sore, she cried: "Enough, O love!
What man shall doubt thou art the son of Jove;
I think thou wilt not die." Then with her hand
She hid her eyes, and trembling did she stand
Until she felt his lips upon her cheek;
Then turning round, with anxious eyes and meek,
She gazed upon him, and some doubtful thought
Up to her brow the tender colour brought.
— "The Doom of King Acrisius," I, 276-77]